Headteachers believe school standards will inevitably suffer if they have to make redundancies. Photograph: Karl Grupe/Getty Images

The day after next week's comprehensive spending review, headteacher Bernadette Lucas (not her real name) will sit down with her staff and present her thoughts about the drastic cuts she believes she will have to make to balance her school's budget.

On the table at the moment are two options. She will either trim spending by 10% across all aspects of the school's overheads, from teaching and support staff budgets to the money devoted to resources, heating and lighting.

Alternatively, she will pursue a two-pronged approach, radically restructuring the school's management team, who command higher salaries than classroom staff, while either increasing class sizes for some subjects or cutting less popular subjects from the curriculum altogether.

It is a tough choice for a secondary school that has thrived in recent years, and appears directly to contradict government claims that "frontline" budgets in classrooms are being protected.

But, at least according to some sources with their ears close to the ground on funding, these kinds of decisions are likely to be faced by heads and governors across England over the coming months, triggered by the spending review. And pupils clearly will be affected.

Lucas says: "Ofsted says we are an outstanding school. Whatever anyone says, if you take money away from a school that is successful, it is not going to run as well as it has in the past."

This kind of talk might provoke sleepless nights among ministers who have made preserving resources to support improvements in classroom performance one of their top priorities, despite the looming cuts across the public sector.

Just before the general election, David Cameron told the BBC's Andrew Marr show that if ministers came to him proposing frontline spending reductions they would be "sent straight back to their department to go away and think again".

In June, a Treasury document setting out the thinking behind the review said that it would ensure that spending was "focused on protecting the quality of...key frontline services".

Yet Lucas already fears that pledge has been broken. "We have already cut back in the past year, including losing some teachers and teaching assistants. Whatever we do now, we will be cutting at frontline services."

Lucas expects to lose £500,000 from her school's budget over the next two years, which is approaching 10%. Some of this relates to cuts that were announced in the summer by the new government. The school has benefited from central grants from Whitehall, which proliferated under Labour, leaving it vulnerable to reductions here.

Lucas's cuts forecast is also based partly on an unofficial "indicative" budget from her local authority for 2011-12, which implies reductions in support services provided to schools once the review is announced.

Back in July, the Observer reported that Michael Gove, the education secretary, had been told by the Treasury to prepare for cuts of between 10% and 20% across his department. Although the government expects reduced waste and cutting administration costs, including quangos, to bear some of the strain, headteachers' leaders say it will be impossible to protect the frontline with this level of reduction.

In terms of direct grants from government, schools across England have already had to grapple with a halving of the money they had been expecting under a "harnessing technology" fund. This is worth £50m nationally, and the new government re-allocated it in June to finance its new free schools policy. Schools such as Lucas's, which had already spent the money, will have to make up the shortfall from elsewhere in their budgets.

Some high-performing secondary schools will also lose funding of £60,000 to £135,000 from the coming year after the government withdrew a grant to reward those offering specialist teaching in more than one subject.

Although it is impossible to know yet the degree to which any budget cuts in next week's spending review plans for 2011-15 will affect education, there is much anecdotal evidence that local authorities, and some schools, are preparing for the worst.

A report on "budget planning" for the next three years from West Sussex council, published in June, warns that schools will need to make "efficiency savings" because of the spending review, and that it plans "briefings and surgeries" for schools managing staffing reductions.

One local authority officer in a London borough says that all schools would need to look at budget cuts, and that support staff might be particularly vulnerable because they might not be considered "frontline". However, three heads spoken to by Education Guardian reject the idea that these employees would be particularly targeted.

Nigel Middleton, a former head and director of the training firm Head Support, which holds advice sessions for hundreds of school leaders every year, says that none of the heads he has talked to expect their budgets to fall by less than 10% over the coming years.

The Association of School and College Leaders believes an average 10% real-terms cut over four years for schools, which equates to a freezing of budgets in cash terms assuming inflation at 2.5% a year, represents a "best case" scenario.

Middleton also predicts that local authority support will be stripped back, with schools likely to be charged for some services that hitherto have been free.

Another head, of a school in Hertfordshire, says: "I have already set my finance manager to finding savings in the 2011-12 budget of 3% to 5%. I am almost certainly going to have to make redundancies.

"I have also set up a restructuring of the senior leadership team. We are losing at least two posts as of September."

She adds that she is also likely to cut staff salaries by up to £5,000, although this cannot be introduced for the next three years under teachers' conditions of service. And the school would have to consider losing both teaching and support staff.

She says: "If you are to lose teaching staff, you either reduce the number of groups that you offer - increasing class sizes - or you cut the number of subjects you provide. So the offer to youngsters becomes a reduced one." Subjects thought to be vulnerable in some schools include languages and humanities.

A deputy head at a London comprehensive says it, like others, is bracing itself for 10% budget cuts over the coming years. Senior staff would be an obvious area for cutbacks, with the school likely not to replace all who leave.

He says: "My head is saying that a lot of schools will not be replacing team leaders when they leave."

Until the review, though, much of this is inevitably conjecture. Two other elements of funding also make predictions of the effect on schools difficult.

First, the government has pledged to introduce a "pupil premium" that will hand extra cash to schools educating pupils from poorer backgrounds. However, it has yet to say how much money will be allocated.

Second, it remains unclear how the budgets for local authority support services to schools will be affected in areas where many institutions move away from authorities to become academies.

In the absence of harder information, pessimism seems to be the order of the day. Middleton says: "I think it's going to be dreadful. When next year comes around, there are going to be significant numbers of redundancies. My worry is that too many school leaders have their heads in the sand, and are not starting to plan early enough."